1. Field of the Invention:
The invention in general relates to communication systems, and particularly to those which require extremely long antennas due to the low frequencies utilized.
2. Description of the Prior Art:
Many communication systems operating at high frequencies utilize a physically large tower as a monopole antenna or to support a monopole antenna. Ideally, the antenna is a quarter wavelength in height (for some midband frequency) for improved efficiency and increased bandwidth.
At very low frequencies, however, a quarter wavelength tower is both physically and economically impractical. For example, a VLF system operating at 61 kHz would need an antenna over 4,000 feet high. A VLF system operating at 17 kHz would dictate a quarter wavelength antenna of close to 15,000 feet. Accordingly, in VLF systems, the monopole antenna arrangement is less than the ideal height. Use of top loading can effectively increase the height of an antenna by a small percentage beyond the actual physical height of the antenna and support tower which in most installations is less than 1,500 feet thus, such antennas are known to be electrically short antennas.
In such systems a counterpoise, that is, an arrangement of radial wires is utilized to effectively shield the earth from the radio frequency (rf) field above it to reduce the magnitude of the current in the earth in the vicinity of the antenna. Very often in a permanent installation, hundreds of wires are utilized as the shield and the wires are electrically connected to a grounding device in the form of a ground rod which penetrates the earth. For a high frequency system, the skin depth in the earth of average conductivity is about 17 feet whereas at very low frequency the skin depth at for example 21 kHz is 114 feet.
For a permanent installation, a construction of a high tower for the antenna as well as the grounding of the hundreds of wires of the counterpoise with a grounding rod over 100 feet long represents no major difficulty. A need exists however for a portable, rapidly deployable VLF communication system which can be set up in the field in a minimal amount of time. A typical permanent installation cannot meet these criteria.